1. Technical Field
The disclosure relates to an apparatus for assisting in driving a vehicle, such as an automobile, and, more particularly, to an apparatus that performs driving assistance in a vehicle including a steering assist mechanism while coordinating control for operating the steering assist mechanism with driver's steering operation.
2. Description of Related Art
In the field of control for driving a vehicle, such as an automobile, there are suggested various types of driving assistance systems or automated driving systems for a vehicle, which control a steering mechanism or acceleration/deceleration mechanism of the vehicle so that a driver is allowed to more easily drive the vehicle. For example, in lane keeping assist (LKA) control, in order to prevent a departure of a moving vehicle from a lane, a driver is warned with a warning buzzer, or the like, when the vehicle is about to depart from the lane. In addition, when radar cruise control is active, speed control for keeping a vehicle speed at a set speed is executed without continuous depression of an accelerator pedal, and the driver is assisted in steering such that the vehicle travels along the lane. In intelligent parking assistant (IPA), steering assist is performed at the time of parallel parking or garage parking. Japanese Patent No. 4173292 describes a configuration that, at the time of a lane change, changes steering reaction force of a steering wheel such that a steering angle is brought close to an optimal steering angle calculated on the basis of surrounding environment information and thus makes it easy for a driver to adjust the steering angle to the optimal steering angle. Japanese Patent Application Publication No. 2000-72021 (JP 2000-072021 A) describes a technique for driving assistance. This technique is configured to determine a target steering angle on the basis of information around a vehicle, increases steering assist force in the same direction as the target steering angle and then reduces steering assist force in a direction opposite to the target steering angle. Furthermore, Japanese Patent Application Publication No. 2010-042741 (JP 2010-042741 A) describes a technique for driving assistance. This technique is configured to calculate a lateral acceleration correction amount for a vehicle to head toward a target position determined by using an image from an in-vehicle camera, detects a driver's steering amount, and, when the steering amount is smaller than a predetermined threshold, decelerates the vehicle and then executes steering control such that the lateral acceleration correction amount is achieved; whereas, when the steering amount is larger than the predetermined threshold, executes steering control and then decelerates the vehicle such that the lateral acceleration correction amount is achieved.
In the above-described driving assistance systems or automated driving systems, more efficient driving of a vehicle is expected to be possible through control based on a system, that is, control based on input by a machine (input of a target set by a machine on the basis of mechanical input, such as surrounding environment information) in comparison with control based on input by a driver (input of steering by a driver or input of a target set on the basis of the input of steering by a driver). A weight reduction of a vehicle is expected from the viewpoint of environmental issues and the need for low fuel consumption and low electric power consumption. However, in the case of a light-weight vehicle, there is a concern about deterioration of stability against cross wind or a road surface disturbance because of reasons, such as small moment of inertia in a yaw direction and deterioration of aerodynamic drag. As a result, usability of driving assistance control based on mechanical input increases. However, if driving assistance control is configured to control the movement of a vehicle on the basis of only mechanical input and not accept driver input (configured to perform complete automated driving), when the movement of the vehicle is different from a movement that is expected from driver's steering operation, the driver can experience a strong feeling of strangeness in that regard. Therefore, in an actual driving assistance system, it is desirable to obtain the advantageous effect of control based on mechanical input while accepting not only mechanical input but also driver input, coordinating both inputs with each other and then allowing both driver input and mechanical input to be reflected in the movement of the vehicle. In terms of this point, in the case of an existing known driving assistance system, such as LKA control and IPA control, if a driver makes an override, such as steering input and accelerator/brake pedal input, while automated driving control is being executed by the driving assistance system, the system stops active control based on mechanical input, so there are many cases where the advantageous effect of control based on mechanical input are not obtained at all.
When the above-described driving assistance system is configured to execute control based on mechanical input while accepting driver input, how to coordinate control based on mechanical input with control based on driver input, that is, how both pieces of control are caused not to conflict with each other, is a challenge. In a state of movement of a vehicle, such as a steering angle, if the difference between a state that control based on mechanical input intends to achieve and a state assumed or expected by a driver is large, the driver experiences a strong feeling of strangeness. As a result, the driver feels that control based on mechanical input is a burden or a nuisance, and, in an extreme case, the driver may steer in a direction opposite to a direction that is intended by mechanical input (in an inefficient direction).
As for one of measures for avoiding the above-described situation as much as possible, that is, for coordinating control based on mechanical input with control based on driver input, it is conceivable to inform a driver of an intention of control based on mechanical input (that is, the direction and/or magnitude of control operation over the movement of a vehicle) and share the intention of control based on mechanical input with the driver. When the driver knows the intention of control based on mechanical input, a situation in which the movement of the vehicle, intended by control based on mechanical input, runs counter to driver's assumption is reduced. It is understood that this reduces a feeling of strangeness that is experienced by the driver. If the driver realizes that driving of the vehicle, which is achieved by control based on mechanical input, is more ideal and understands an intention of control based on mechanical input, the reliability of the system increases, and the degree of reliance on the system increases, with the result that the driver performs input so as to follow control based on mechanical input, that is, control based on mechanical input and control based on driver input are executed in a coordinated manner.
It is conceivable that a driver is informed of an intention of control based on mechanical input by display (visual sense) or sound (audio sense). However, when assistance of the driving assistance system is particularly for steering of the vehicle, the driver may be informed through a steering wheel gripped by the driver. In the case of driving assistance in steering the vehicle, when the driving assistance system is configured to, at the time when additional steering torque (steering assist torque) is applied in a direction in which the steering angle is controlled in order to cause the vehicle to follow an ideal path, accept driver input, that is, steering torque that is applied through the steering wheel by the driver (driver's steering torque), for steering of the vehicle and then control the steering angle by using the steering assist torque and the driver's steering torque, a steering device for a vehicle, in which the steering wheel and wheels are mechanically directly coupled to each other, is mounted (alternatively, the steering wheel and the wheels may be mechanically directly coupled to each other only when the system accepts the driver's steering torque). With this configuration, the steering assist torque is transmitted to the steering wheel because the steering wheel and the wheels are mechanically directly coupled to each other, so the steering wheel serves as an informing unit that informs the driver of operation that control based on mechanical input intends to perform. Thus, the driver is allowed to feel the direction and magnitude of steering assist torque via a hand gripping the steering wheel. When the driver performs steering such that the vehicle appropriately follows to approach an ideal path that is a target of control based on mechanical input with the aid of the direction and magnitude of steering assist torque, it is expected to achieve a target state of control based on mechanical input. In this manner, the steering device in which the steering wheel and the wheels are mechanically directly coupled to each other is mounted on the vehicle, and the vehicle is configured such that the driver is allowed to sense steering assist torque through the steering wheel. In this state, when the direction and magnitude of steering assist torque that represents an intention of control based on mechanical input are transferred to the driver, the driver understands and relies on the intention of control based on mechanical input, and performs steering so as to follow the direction and magnitude of steering assist torque. As a result, in control over the steering angle, control based on mechanical input through steering assist torque and control based on driver input through driver's steering torque that the driver applies through the steering wheel are executed in a coordinated manner and do not conflict with each other.
Incidentally, there are individual differences in driving characteristic or driving skill level among drivers, and, even in the case of the same driver, the driving characteristic or the driving skill level can change depending on whether the driver is accustomed to driving on the course of a traveling road, a time period of driving, the length of time, physical condition, and the like. The extent of accuracy in the case where a driver senses steering assist torque through the steering wheel and then applies steering torque following the steering assist torque depends on the driving characteristic of the driver, particularly, the driving skill level of the driver. That is, when driving assistance for drivers of various driving skill levels is considered, adequate extent of assistance differs depending on how high the driving skill level is. Therefore, in the above-described driving assistance made by applying steering assist torque, it is desirable that the extent or mode of the driving assistance be allowed to be changed or adjusted depending on the driving characteristic or driving skill level of a driver.
When the extent or mode of applying steering assist torque is changed depending on the driving characteristic or driving skill level of a driver, it is required to understand the driving characteristic or driving skill level of a driver. In terms of this point, in the case of the configuration in which a driver senses the direction and magnitude of steering assist torque that appears on the steering wheel and applies steering torque following the steering assist torque, it is possible to evaluate the driving characteristic or driving skill level of the driver on the basis of a deviation between the steering assist torque and the driver's steering torque. For example, as the driving skill level of a driver becomes higher, the manner of applying steering torque makes it possible to bring an actual driving state to a state closer to ideal driving that is a target of control based on mechanical input, and it is presumable that the manner of applying steering torque accurately follows the direction and magnitude of steering assist torque and reduces a deviation between steering assist torque and driver's steering torque. Therefore, a value that is determined on the basis of a deviation between steering assist torque and driver's steering torque can be used as an index value of driving skill level. Such findings are utilized in the invention.